I would say they're not liable, because it's not like they were negligent, they were trying to heal their child but in a different way. The faith healing didn't harm their child directly, it was the lack of medical attention.

"Freedom of religion?" The child didn't have a religion, but that doesn't mean the child didn't have rights. I've heard people say metaphorically that they'd go through hell for their kids. Here we have parents that WOULDN'T. They placed their own "rights" over the life of their child, and I have no sympathy at all for them. For the child, yes, but not for those abusive parents.